1. Field of the Invention
The present invention relates to a method and apparatus for controlling a lifting magnet of a materials handling machine for which the source of electrical power is an AC power source.
2. Prior Art
Lifting magnets are commonly attached to hoists to load, unload, and otherwise move scrap steel and other ferrous metals. For many years, cranes were designed to be powered by DC sources, and therefore systems used to control lifting magnets were designed to be powered by DC as well. When using a hoist, due to the nature of the overhauling load, the torque and speed of the hoist motor need to be controlled. The traditional approach was to control the DC motor torque and speed by selecting resistors in series with the DC motor field and armature windings by means of contactors. In recent years, with the advance of electronic technology in the field of motor control, systems used to control lifting magnets, namely cranes, are now designed to be powered by AC sources. Cranes are now equipped with adjustable-frequency drives, commonly referred to as AC drives, which can accurately control the speed and torque of AC induction motors. The use of AC supplies removes the costs of installing and maintaining large AC-to-DC rectifiers, of replacing DC contactor tips, and of maintaining DC motor brushes and collectors. However, in order to use a lifting magnet on one of the new AC supplied cranes, a rectifier needs to be added to the crane. The rectifier that needs to be added to the crane is generally composed of a three-phase voltage step-down transformer connected to a six-diode bridge rectifier. The rectifier that is added to the crane is either mounted on the crane itself, where the rectifier becomes a weight constraint and an obstruction, or the rectifier is mounted elsewhere in the plant, in which case additional hot rails are required along the bridge and trolley in order for the DC electrical power to reach the DC-supplied magnet controller.
While lifting magnets have been in common use for many years, the systems used to control these lifting magnets remain relatively primitive. During the “Lift”, a DC current energizes the lifting magnet in order to attract and retain the magnetic materials to be displaced. When the materials need to be separated from the lifting magnet, most of the controllers automatically apply a reversed voltage across the lifting magnet for a short period of time to allow the consequently reversed current to reach a fraction of the “Lift” current. The phase during which there is a reversed voltage applied across the magnet is known as the “Drop” phase, during which a magnetic field in the lifting magnet of the same magnitude but in an opposite direction of the residual magnetic field is produced such that the two fields cancel each other. When the lifting magnet is free of residual magnetic field, the scrap metal detaches freely from the lifting magnet. This metal detachment is known as a “Clean Drop”.
Some control systems operate to selectively open and close contacts that, when closed, complete a “Lift” or “Drop” circuit between the DC generator and the lifting magnet. At the end of the “Lift”, which is called the “discharge” and at the end of the “Drop”, which is called the “secondary discharge”, these systems generally use either a resistor or a varistor to discharge the lifting magnet's energy. The higher the resistor's resistance value or varistor breakdown voltage, the faster the lifting magnet discharges, but also the higher the voltage spike across the lifting magnet. High voltage spikes cause arcing between the contacts. In addition, fast rising voltage spikes also eventually wear out the lifting magnet insulation, and the insulation of the cables connecting the lifting magnet to the controller. To withstand these voltage spikes, generally in the magnitude of 750 V DC with systems using DC magnets rated at 240 V DC, the lifting magnet, cables, and the control system contacts and other components need to be constructed of more expensive materials, and also need to be made larger in size.
Lifting magnets are rated by their cold current (current through the magnet under rated voltage, typically 250V DC, when the magnet temperature is 25° C.). These lifting magnets are designed for a 75% duty cycle (in a 10 minute period the magnet can have voltage applied at 250V DC for 7 minutes 30 seconds and the remaining 2 minutes 30 seconds the magnet must be off for cooling or the magnet will overheat). Today, magnet control systems are limited by the rectified DC voltage supplying the magnet control (typically 250-350V DC). These systems control the voltage to the magnet and as the magnet heats up, the resistance rises and the current drops. As a magnet heats up, the magnet loses 25-35% in lifting capacity because the resistance of the wire increases and the current through the lifting magnet decreases.